Halogen containing polymers, especially chlorine containing polymers, have attained a high degree of commercial significance and use. Polyvinyl chloride (PVC), in particular, is widely used in packaging, siding, pipe and extruded shapes. Such large scale and diverse use of the halogen containing polymers, as exemplified by PVC, is the result, at least in part, of the stabilization of such polymers against degradation caused by light and/or heat. PVC is known to degrade upon prolonged exposure to light and/or heat during processing and use, with attendant darkening or change of color and loss of physical properties (e.g. tensile, flexural and impact strengths). Such degradation detracts from the usefulness of PVC and imposes restrictions on the conditions and apparatus for processing and manufacturing PVC articles. Thus, for example, the service life of a PVC article may be significantly limited, the conditions under which the PVC article is used severely restricted and the manufacturing conditions very narrow.
One particularly troublesome form of degradation of halogen containing polymers occurs when they are processed into articles by methods employing heat to melt or soften the polymer. Color changes can occur in the polymer during the first few minutes (e.g. about 1 to 10 minutes) of processing at high processing temperatures (e.g. about 175.degree. C. to 200.degree. C.). This color change during the first several minutes of exposure to high processing temperatures is commonly referred to as early color or early discoloration. Avoidance of such early color or early discoloration is notably important in a number of applications (e.g. plastic pipe) and is particularly important where white or light colored products are to be made. It is, of course, also important to prevent or reduce discoloration and deterioration of the organic polymer during extended exposure to high processing temperatures, as may be encountered in some processes or fabricating methods. This extended exposure to high processing temperatures can lead to a sudden catastrophic degradation of the polymer which transforms the polymer into a pitch-like abrasive material and can, especially in the case of halogen-containing organic polymers, cause the formation of corrosive materials, e.g. HCl, inside the fabricating equipment. This abrasion and corrosion can ruin the inner, highly-polished surfaces of the fabricating equipment and render the equipment essentially useless.
To prevent or reduce the discoloration and deterioration of halogen-containing polymers during processing at elevated temperatures and during exposure of the fabricated product to elevated temperatures under use conditions, the art has added various materials, known as stabilizers, to the polymers. Most notable among these stabilizers have been the organotin stabilizers. These organotin stabilizers have been found to be particularly effective in the prevention of early discoloration of halogen containing polymers. However, while the organotin stabilizers are capable of lessening or preventing early discoloration, they have in recent years become increasingly expensive, with the result that in relatively low cost products such as pipe the cost of such stabilizers may be difficult to bear.
Attempts have been made to replace the organotin stabilizers, either in part or altogether, with lower cost materials. Various combinations of organotin compounds with other organometallic compounds have thus been developed. One such combination, disclosed in Japanese Kokai 77 37,956 by Minagawa et al., contains at least one alkyl tin compound selected from monoalkyl and dialkyl tin compounds, and at least one mercaptide, salt or ester of mercaptocarboxylic acid, or salt or ester of maleic acid with metals selected from Mg, Ca, Sr, Ba, Zn, Sn or Zr. As the alkyl tin compounds there are disclosed such compounds as, for example, dimethyltin oxide, dimethyltin sulfide, dibutyltin bis(isooctylmercaptoacetate) and monobutyltin tris(isooctylmercaptopropionate). The metal mercaptides, salts or esters of mercaptocarboxylic acids include such compounds as, for example, zinc bis(isooctyl thioglycolate). The complete elimination of organotin compounds was attempted in Japanese Kokai 75 89,451 to Shinkawa. Disclosed are stabilizers for halogen-containing resins containing organozinc compounds and organic acid alkali or alkaline earth metal salts. The organozinc compounds have the formula: EQU Zn(SX)(Y)
where SX is a mercaptan or mercaptate residue, and Y is the same as SX or an organic acid residue. An example of such organozinc compounds is ##STR1##
Likewise, U.S. Pat. No. 2,723,965 to Leistner et al. discloses stabilizers for halogen containing polymers which are organometallic compounds having the formula: EQU R--S--Y--COOMe
where R is aryl or alkyl, Y is alkylene, and Me is any metal other than an alkali metal, examples of which include the alkaline earth metals, cadmium, lead, zinc and tin.